Backdraft dampers are used to prevent the backdraft of air in various industrial and commercial heating, ventilating and air conditioning (HVAC) systems.
Such dampers typically comprise an outer frame sized to either fit into a specified opening or to cover a specific opening, in various environments. The damper blades are movable from an open position in which air is permitted to flow through the damper frame in one direction, and a closed position blocking the flow of air through the damper frame in the other direction, in order to prevent the contamination of air within a premises and/or the ingress of thermally unfavourable air (warm or cold) into a thermally controlled premises.
A backdraft damper must work automatically under the force of air, flowing either in the intended (outflow) direction, in which the airflow maintains the backdraft blades in an open condition, or in the reverse (backdraft) direction in which the loss of outflow air causes the backdraft blades to move to the closed position under the influence of gravity, and the backdraft maintains the blades in the closed position for the duration of the backdraft current. In order to ensure this, the blades must be biased to the closed position by gravity. However, this means that some of the force of the air flowing in the outflow direction is always sacrificed in order to maintain the damper blades in the open position, which reduces the airflow of the outflow current. HVAC systems are typically carefully designed to distribute air evenly about a premises, and this reduction in airflow can have the effect of skewing the pressure distribution to some flow-paths over others, reducing the intended airflow rates to some parts of the premises.
One solution to this is to try and balance the blades about their respective pivot rods so that little force is required to open them. However, this can cause inadvertent leakage in the backdraft direction, resulting in lower efficiency where the backdraft damper is providing thermal protection, and in situations where the backdraft damper is preventing the potential ingress of toxic or noxious gasses can result in a serious risk to occupants of the premises.
It would accordingly be advantageous to provide a backdraft damper having blades which are biased to the closed position with sufficient force to prevent the blades from remaining open when the outflow current is disrupted, but which can be opened with a relatively low force without impeding the airflow through the damper and thus without losing pressure to maintaining the damper in the open position.